Rotary line stretcher

ABSTRACT

A COMPACT, CONSTANT IMPEDANCE LINE STRETCHER IS CAPABLE OF PRODUCING A LARGE, CONTINUOUS CHANGE IN ELECTRICAL LENGTH. AN ELECTRODE, SECURED TO THE SURFACE OF A BODY OF DIELECTRIC MATERIAL, IS MOVABLE WITH REFERENCE TO A STATIONARY ELECTRODE. IN ONE EXTREME POSITION OF THE MOVABLE ELECTRODE, AIR COMPRISES VIRTUALLY THE ONLY DIELECTRIC BETWEEN THE ELECTRODES. IN AN OPPOSITE EXTREME POSITION, A LARGE PART OF THE ELECTRIC FIELD FLUX BETWEEN THE ELECTRODES PASSES THROUGH THE DIELECTRIC BODY. THROUGHOUT INTERMEDIATE POSITIONS, THERE IS A GRADUAL CHANGE IN THE PROPORTION OF THE FLUX PASSING THROUGH THE DIELECTRIC BODY.

Feb.2, 1971 J. 1. SMITH I ROTARY LINE STRETCHER Filed Nov. 22, 1968 A TTORNE Y 3,560,890 ROTARY LINE STRETCHER John 1. Smith, Morris Township, Morris County, N.J.,

assignor to Bell Telephone Laboratories, Incorporated,

Murray Hill and Berkeley Heights, N.J., a corporation of New York Filed Nov. 22, 1968, Ser. No. 778,149 Int. Cl. H03h 7/30 US. Cl. 333-31 9 Claims ABSTRACT OF THE DISCLOSURE A compact, constant impedance line stretcher is capable of producing a large, continuous change in electrical length. An electrode, secured to the surface of a body of dielectric material, is movable with reference to a stationary electrode. In one extreme position of the movable electrode, air comprises virtually the only dielectric between the electrodes. In an opposite extreme position, a large part of the electric field flux between the electrodes passes through the dielectric body. Throughout intermediate positions, there is a gradual .change in the proportion of the flux passing through the dielectric body.

GOVERNMENT CONTRACT The invention herein claimed was made in the course of, or under contract with The Department of the Army.

BACKGROUND OF THE INVENTION This invention relates to the wave transmission art and more particularly to a line stretcher capable of changing the electrical length of a transmission line.

Line stretchers of the prior art generally operate on principles requiring either the physical length of a portion of the line or one of its physical cross section dimensions to be varied. A line stretcher which is varied in physical length can be made to have a substantially constant impedance but none of those which vary a cross section dimension is known to maintain a constant impedance. The variable physical length lines are objectionable in that they require considerable space and often cannot be used with rigid transmission lines while those which vary in cross section dimension, while generally shorter, are objectionable because their characteristic impedance changes. A compact, constant impedance line stretcher capable of a large change in electrical length is needed in many microwave applications, for example, those in which a plurality of lines either require length equalization or their relative lengths must be adjusted to meet some relative phase requirement. The present invention has the desirable features of compactness, constant impedance and large changes in electrical length.

SUMMARY OF THE INVENTION This invention comprises a compact, constant impedance line stretcher in which a movable electrode is mounted on the surface of a body of dielectric material. A stationary electrode is positioned opposite the movable electrode so that, in one extreme position of the movable electrode, the two electrodes are directly opposite each other with air comprising virtually the only dielectric between them. 'In another extreme position of the movable electrode, a large part of the electric field flux between the electrodes passes through the body of dielectric material. Throughout intermediate positions, there is a gradual change in the proportion of the total field flux passing through the body of dielectric material. Opposite ends of the two electrodes are connected to a transmission line through suitable connectors.

"United States Patent O BRIEF DESCRIPTION OF THE DRAWINGS The invention may be better understood by referring to the accompanying drawings in which:

FIG. 1 discloses the principal features of a preferred embodiment of the invention; and

FIGS. 2 and 3 show two extreme positions of the electrodes.

DETAILED DESCRIPTION The embodiment shown in FIG. 1 comprises a substantially cylindrical rotor 1 of dielectric material included within a cylindrical housing or outer conductor 2. Rotor 1 is supported at each of its ends by a bearing 3 consisting of a simple steel shaft entering a bearing formed in the inner end of the center conductor 4 of a coaxial connector. This bearing support should provide free rotation for the rotor 1 as well as a good electrical coupling between the center conductor 4 and the shaft 3. A variety of other bearing supports, well known in the art, may be substituted for the one shown for illustrative purposes and one such support is exemplified by United States Pat. 2,700,137 granted Jan. 18, 1955, to G. L. Ragan.

A conductive strip 8 is mounted on a surface of the rotor 1 as shown in all three figures, the ends of which are brought over the end of the rotor and electrically secured to the shaft 3 at each of the ends of the rotor. It is to be understood that the bearing and coaxial connector construction shown in partial section at the right side of this drawing is duplicated at the left end. The right end of the outer conductive cylinder 2 is closed by a removable end wall 9 and secured thereto by conventional means, not shown. The left end of the cylinder 2 is preferably closed by an end wall 9A made permanently integral therewith. It is also convenient to form the outer conductors l0 and 10A of the coaxial connectors integral with end walls 9 and 9A, respectively. A stationary electrode 11 extends from the inner surface of the outer conductive cylinder 2 toward the rotor 1, its position being best understood by referring to both FIGS. 1 and 2. The stationary electrode 11 is conductively connected to cylinder 2 and, consequently, to the outer conductors 10 and 10A of the coaxial connectors.

The coaxial connectors may be of conventional construction except for the bearing supports shown in FIG. 1 and may comprise a pair of insulating supports 7 forming a tight fit with the inner conductor 4, thereby rigidly holding this inner conductor in place. The outer ends of these connectors are adapted for connection to a coaxial cable through the conventional means of a tapered portion 5 on the inner conductor and a threaded portion 6 on the outer conductor.

The rotor may be readily changed in its angular position about its bearing supports by means of a knob 16 attached to a gear sector 15 through a shaft 17. Knob 16 may be knurled for rotation by hand or may contain a screwdriver slot or a socket for a conventional set screw wrench. Spacer 18 is inserted between the inner surface of end wall 9 and gear sector 15 to maintain the axial position of shaft 17. Gear sector 15 contains a number of gear teeth 14 which cooperate with gear teeth 13 formed in the end of rotor 1, best shown in FIG. 2.

FIG. 3 shows the rotor 1 rotated in one of its extreme positions so that movable electrode 8 has a surface directly opposite a surface of the stationary electrode 11 so that the intervening space 12A contains essentially only air. Under these circumstances, the dielectric flux passes primarily only between the two electrode surfaces through an air path and does not include any substantial amount of the dielectric material of rotor 1. When the rotor is moved through an angle of approximately as shown in FIG. 2, the movable electrode 3 is moved away from a the stationary electrode 11 so that the fiux path through gap 12 toward electrode 3 necessarily includes a substantial amount of the dielectric material of rotor 1. FIG. 2 shows a substantial air gap 12 between rotor 1 and the stationary electrode 11 but in actual practice this gap is preferably made small. It will be understood that in intervening positions of the rotor varying proportions of air and solid dielectric material are included in the gap between the surface of stationary electrode 11 and the movable electrode 8.

To better understand the nature and operation of the present invention, a brief discussion of its theory will be given. This line stretcher takes advantage of the very large difierence in the phase velocity between ditferent propagation conditions of waves in an inhomogeneous dieelectric medium. This large difference comes about because the flux lines under one condition of propagation can be made more highly concentrated within the dielectric between electrodes 8 and 11 than in another condition of propagation. The phase velocity for waves in a principal mode in an inhomogeneous dielectric medium can be expressed as:

o( 1 k) where:

V =velocity of light.

C capacitance between the electrodes in the absence of any solid dielectric support.

C capacitance between the electrodes in the presence of the solid dielectric support.

The characteristic impedance Z of such a structure for a principal mode is:

The electrical length 6 of a transmission line having a physical length L can be shown to be:

where:

x =free space wavelength. V=propagation velocity through the line stretcher.

If the rotor 1 is in the position shown in FIG. 3 where no substantial amount of the solid dielectric material is contained in the fiux path between the tWo electrodes, the velocity V shown in expression 3 approximates the velocity of light. However, as the rotor is rotated toward the position shown in FIG. 2 capacitance C; reduces and, because an increasing amount of the solid dielectric material is included in the flux path, capacitance C increases so that the velocity V, as shown by expression 1), becomes gradually less than the velocty of light. It is assumed for the moment that the characteristic impedance Z is kept constant as indeed it can be. Since the velocity V through the line stretcher changes as the position of rotorl is changed, the change in electrical length A0 for any two positions of the rotor can be obtained from expression (3) and will be:

where the subscript numerals refer to the two rotor positions.

Expression (2) indicates that a constant impedance between electrodes 8 and 11 may be maintained providing the product of the two capacitances C and C is kept constant. By shaping the rotor approximately as shown in FIG. 2, a reasonably close approximation of a constant capacitance product can be achieved, although it is obvious that the capacitance ratio will change greatly. The proper shape of the dielectric rotor can be achieved by the well known engineering method of plotting the dielectric flux for several rotor positions. This method is described in Chapter 6, Section 19, of Weber: Electromagnetic Theory, Static Fields, and Their Mapping, pp. 201-204; Dover Publications, 1965.

To avoid undesired reflections, the characteristic impedance should be kept constant throughout the length of the line stretcher. The coaxial connectors 4, 10, 4A and 10A should, of course, have the same characteristic impedance as the line to which they are to connect and the section comprising electrodes 8 and 11 should be constructed to have this same impedance for all rotor positions. The overlapped ends of electrode strip 8 and end walls 9 and 9A should also have this impedance which obviously remains constant for all rotor positions.

By way of a practical example, if a fifty-ohm line stretcher is made in accordance with this invention to operate in L band and has a length of four inches, a maximum change in electrical length of 400 degrees may readily be obtained. It should be noted that this change in electrical length is obtained without any change in physical length of the line stretcher. It is also evident that the construction is simple, very compact and convenient to use.

Having described the invention with reference to a specific physical embodiment, it will be evident to those skilled in this art that various modifications may be made employing the principles of this invention without departing from its scope.

What is claimed is:

ll. A variable length line stretcher comprising first and second electrodes having capacitance between them, the first of said electrodes being secured to a solid dielectric support and arranged for motion relative to said second electrode, and means for continuously moving said solid dielectric support from one extreme position where air comprises essentially the sole dielectric between said electrodes to a second extreme position Where virtually all the electric flux between the electrodes passes through a portion of said solid dielectric support.

2. The combination of claim 1 and a coupling means electrically coupled to said two electrodes for inserting said line stretcher in a line, the length of which is to be changed.

3. The combination of claim 2 wherein said coupling means comprises a pair of coaxial connectors.

4. The combination of claim 1 wherein said support of solid dielectric material is of substantially cylindrical shape and has a pair of hearings on its ends for permitting rotation thereof about its principal axis.

5. The combination of claim 4 and a conductive enclosure surrounding said two electrodes and mechanically and electrically secured to said second electrode, and said means for continuously moving said first electrode comprises a means mechanically coupled to said support of solid dielectric material to permit motion thereof on its bearings from outside said enclosure.

6. A rotary type, constant impedance line stretcher comprising a movable conductive electrode secured to the surface of a rotatable support of dielectric material, a stationary conductive electrode having a surface shape and size substantially the same as those of a surface on said movable conductive electrode, said two electrodes being positioned so said surfaces may oppose each other with air comprising essentially the only dielectric in the flux path between them, means for rotating said support so the surface of said movable electrode is gradually moved from its position opposite the surface of said stationary electrode until a substantial amount of the dielectric material of said support enters the flux path between said two electrodes, and means coupled to said two electrodes to provide external connections thereto.

7. The combination of claim 6 wherein said means coupled to said two electrodes comprises coaxial connectors having inner and outer conductors coupled, respectively, to said movable electrode and to said stationary electrode.

8. A rotary type, constant impedance line stretcher comprising a substantially cylindrical rotor of dielectric material having a conductive bearing support at each of its ends providing an axis of rotation for said rotor, each of said supports adapted for electrical coupling to the inner conductor of a coaxial cable, a strip of conductive material forming an electrode on the surface of said rotor extending parallel to the rotor axis and connected at its ends to said bearing supports, a hollow cylindrical conductor surrounding said rotor, said cylindrical conductor being closed at each of its ends with conductive material arranged for connection to the outer conductor of said coaxial cable, a stationary electrode extending radially from the inner wall of said cylindrical conductor having a length and width substantially equal to that of the electrode on said rotor, and means for adjusting the angular position of said rotor about its axis.

9. The combination of claim 8 and a coaxial connector on each of the closed ends of said hollow cylindrical References Cited UNITED STATES PATENTS 4/1958 Casal 33331 1/1962 Kern et al. 333-31 10 ELI LIEBERMAN, Primary Examiner S. C-HATMON, JR., Assistant Examiner US. Cl. X.R.

conductor, the connector at each end having an inner 15 333-73, 81 

